feat: add gamepad support to escoria-ui-simplemouse (#518)

This commit starts by introducing a new action, `ESC_UI_PRIMARY_ACTION`,
that is used to represent a "primary action" from an input device, such
as a left-click on a mouse or the press of the primary button on a
gamepad. Although this action is not added to `InputMap` by default in
`escoria-core`, `_unhandled_input()` in `esc_item.gd` is updated to look
for it.

The other portion of this commit is an update to
`addons/escoria-ui-simplemouse/game.gd` that adds a gamepad mapping to
the `InputMap` for both `ESC_UI_PRIMARY_ACTION` as well as another new
action `ESC_UI_CHANGE_VERB_ACTION`. These actions are mapped to X and Y
on an XBox controller, respectively.

Note that `game.gd` is also updated to implement `_process()` such that
moving `JOY_AXIS_0` and `JOY_AXIS_1` on "gamepad 0" will move the
cursor around on the screen as the mouse would.

Overall, this makes the game fairly playable with a gamepad, though
admittedly a mouse is still currently required to click the "New game"
button on the initial menu because `BaseButton::gui_input()` appears to
be hardcoded to check for mouse events exclusively:

godotengine/godot@a09814e/scene/gui/base_button.cpp#L55-L81

addons/escoria-core/game/inputs_manager.gd

@@ -21,6 +21,10 @@ const SWITCH_ACTION_VERB = "switch_action_verb"
 # Input action for use by InputMap
 const ESC_SHOW_DEBUG_PROMPT = "esc_show_debug_prompt"
 
+# Input action for use by InputMap that represents a "primary action" from an
+# input device, such as a left-click on a mouse or the X button on an XBox
+# controller
+const ESC_UI_PRIMARY_ACTION = "esc_ui_primary_action"
 
 # The current input mode
 var input_mode = INPUT_ALL